1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of the Related Art
Recently, with improvement of engine performance, spark plugs are required to have further extended service life and further improved resistance to contamination. For example, a so-called creeping discharge spark plug is a spark plug for an internal combustion engine having improved contamination resistance. The creeping discharge spark plug is configured such that a spark generated at a spark discharge gap propagates along a surface of an insulator; i.e., in the form of creeping discharge, at all times or depending on particular conditions. A semi-creeping discharge spark plug, which is one type of the creeping discharge spark plug, includes a center electrode, an insulator surrounding the center electrode, and a ground electrode having at its end a discharge surface, which is disposed to face a side surface of the center electrode. The tip end portion of the insulator is disposed to have a positional relationship with the center electrode and the ground electrode such that the end portion of the insulator is located between the center electrode and the discharge surface of the ground electrode (i.e., located in the spark discharge gap). In such a semi-creeping discharge spark plug, when a spark travels along the tip end surface of the insulator, aerial discharge occurs between the surface of the insulator and the discharge surface at the tip end of the ground electrode.
When a spark plug is used for a long period of time at a low temperature not higher than 450xc2x0 C.; for example, during predelivery, the spark plug becomes xe2x80x9csootedxe2x80x9d or xe2x80x9ccovered with fuel.xe2x80x9d In such a state, the insulator surface is covered with a conductive contaminant, such as carbon, which causes defective operation. However, in the case of the above-described creeping discharge spark plug, while spark discharge creeps across the surface of the insulator, an adhering contaminant is burned off at all times, and thus the creeping discharge spark plug exhibits improved resistance to contamination as compared with a parallel-electrode-type spark plug.
Meanwhile, such a creeping discharge spark plug involves frequent occurrence of a spark which creeps across the surface of an insulator, and thus tends to suffer so-called channeling, or a phenomenon whereby the surface of an insulator is abraded and grooves are formed on the surface. Progress of channeling is apt to impair heat resistance or reliability of a spark plug, and channeling is particularly apt to occur during high-speed or heavy-load operation. With the recent trend toward high engine output, there has been demand for spark plugs of excellent durability, and a requirement for prevention or suppression of channeling is becoming stricter.
In some cases, the center electrode of a spark plug is formed of an Ni-base heat-resistant alloy in order to improve heat resistance. However, since the Ni-base heat-resistant alloy contains a relatively large amount of a secondary component such as Cr or Fe, thermal conductivity decreases considerably, depending on the composition. As a result, the heat-transfer performance of the electrode is lowered with resultant acceleration of consumption of the electrode or consumption of a noble-metal discharge portion formed on the electrode. Thus, when the spark plug is used in an environment in which the electrode temperature is prone to rise; i.e., during high-speed, heavy-load operation, the service life of the plug is shortened.
It is therefore an object of the present invention to provide a spark plug whose center electrode has improved heat-transfer performance, which has improved durability against electrode consumption and excellent contamination resistance, and which hardly causes channeling.
In order to achieve the above object, the present invention provides a spark plug of a first structure comprising:
a center electrode:
an insulator surrounding the center electrode; and
a ground electrode positioned relative to a tip end portion of the insulator and a tip end portion of the center electrode such that a spark discharge gap is formed between the ground electrode and the tip end portion of the center electrode, and creeping spark discharge along a surface of the tip end portion of the insulator can occur at the spark discharge gap, wherein
an electrode base material which forms at least a surface layer portion of the center electrode is made of an Ni alloy having a coefficient of thermal conductivity of 17 to 30 W/mxc2x7K, the Ni alloy containing Ni as a predominant component and an element (hereinafter referred to as an xe2x80x9cNTC elementxe2x80x9d), as a secondary component, which element can form an oxide semiconductor having a resistivity of negative temperature coefficient (hereinafter also referred to as an xe2x80x9cNTC oxide semiconductorxe2x80x9d).
When the center electrode is formed of an Ni alloy containing an NTC element as a secondary component and having a coefficient of thermal conductivity falling within the above-described range, a layer containing an NTC oxide semiconductor and serving as a corrosion suppression layer is easily formed on the surface of the tip end portion of the insulator. Thus, corrosion of the surface of the tip end portion of the insulator due to creeping spark discharge can be suppressed effectively, and the electrode can have improved heat transfer property, so that durability in terms of electrode consumption can be greatly improved.
The above-described corrosion suppression layer decreases the discharge voltage at the spark discharge gap. When this effect is utilized, suppression of consumption of the electrode (or a noble-metal consumption-resistant portion formed on the electrode) and further reduction of channeling can be attained. Moreover, in order to enable creeping spark discharge, the shortest distance between the insulator and the ground electrode is preferably made shorter than the shortest distance between the center electrode and the ground electrode.
In the first structure of the present invention, two or more ground electrodes can be disposed around the center electrode. This configuration enables sparks to be generated at positions distributed along the circumference of the insulator, and therefore is advantageous in suppressing formation of deep channels.
The spark plug having the first structure according to the present invention may be embodied as follows. That is, a plurality of ground electrodes are disposed around the center electrode; and at least one ground electrode among them is a semi-creeping ground electrode which is disposed such that its end surface faces a side surface of the center electrode, while at least a portion of the tip end portion of the insulator is interposed therebetween to thereby form a semi-creeping discharge gap between the end surface of the semi-creeping ground electrode and the side surface of the center electrode. In this structure, since the end surface of the ground electrode and the side surface of the center electrode face each other, while sandwiching at least a portion of the tip end portion of the insulator, creeping spark discharge along the surface of the insulator occurs more frequently, so that the spark plug can have excellent contamination resistance. In conventional spark plugs, the above-described structure is not necessarily desirable from the viewpoint of suppression of channeling of the insulator. However, in the present invention, since the center electrode is made of an Ni alloy containing the above-described NTC element as a secondary component as described above, a spark plug can be realized which exhibits excellent channeling resistance even when creeping spark discharge frequently occurs. Further, the distance E between the tip end surface of the insulator and the rear-side edge of the end surface of the ground electrode; i.e., the distance of overlap between the tip end surface of the ground electrode (semi-creeping ground electrode) and the side surface of the tip end portion of the insulator along the axis of the center electrode, is preferably set to 0.2 mm or more. In this case, the effect of the insulator 3 for blocking a discharge passage and thus the channeling suppressing effect become more remarkable.
In the above-described structure, one of the plurality of ground electrodes may be a parallel ground electrode which is disposed such that a side surface of a tip end portion of the ground electrode faces, in parallel, the tip end surface of the center electrode to thereby form a parallel aerial discharge gap. In this case, a parallel aerial discharge gap similar to that found in a so-called parallel electrode spark plug is formed between the side surface of a tip end portion of the parallel ground electrode and the tip end surface of the center electrode; and a semi-creeping discharge gap is formed between the tip end surface of the semi-creeping ground electrode and the side surface of the center electrode. When the size of the parallel aerial discharge gap is rendered greater than that of the semi-creeping discharge gap, sparks are generated more easily at the parallel aerial discharge gap in an ordinary state; and when the tip end surface of the insulator is contaminated, sparks are generated more easily at the semi-creeping discharge gap. Sparks concentrate at the parallel aerial discharge gap to a high degree, and the frequency of spark discharge at a projected position is high. Therefore, ignition performance can be further enhanced.
The spark discharge gap having the first structure according to the present invention may be embodied as follows. That is, a center electrode is disposed in an insulator such that a tip end portion of the center electrode projects from the insulator; and a cylindrical metallic shell is provided to surround the insulator. A base end portion of a ground electrode is welded to an end portion of the metallic shell; and a tip end portion of the ground electrodes is bent toward the center electrode such that an end surface of the ground electrode faces a side surface of the projecting tip end portion of the center electrode to thereby form a first gap, and an inner surface of the tip end portion of the ground electrode faces the tip end surface of the insulator to thereby form a second gap, which is smaller than the first gap. The spark plug is of a so-called intermittent creeping discharge type. Before contamination does not considerably proceed, spark discharge occurs at the first gap, which is advantageous from the viewpoint of ignition performance; and when contamination has proceeded, the resistivity of the surface of the insulator decreases, and spark discharge at the second gap starts. In other words, the progress of contamination at the surface of the insulator is detected automatically, and intermittent spark discharge is caused to occur at the second gap, so that contaminant deposit is burnt out. Thus, a creep discharge spark plug is realized which has excellent contaminant resistance, while maintaining ignition performance at the time of ordinary spark discharge. Moreover, since sparks are not produced by means of creeping discharge at all times, the above-described configuration is advantageous from the viewpoint of channeling suppression.
In the above-described structure, as shown in FIG. 5, when the side, with respect to the axis of the center electrode, on which the tip end surface of the center electrode is located is referred to as the front side, and the side opposite the front side is referred to as the rear side, the distance h between the rear-side edge of the end surface of the ground electrode and the tip end surface of the insulator as measured along the axial direction is preferably set to 0.3 mm or more. The distance h determines the size of the second gap g2 for creeping discharge. When the distance h is set to a relatively large value, the channeling resistance can be improved further. However, when the distance h exceeds 0.7 mm, the discharge voltage at the second gap becomes excessively high, and the function as an intermittent creeping discharge spark plug becomes insufficient in some cases. Therefore, the distance h is preferably set to 0.7 mm or less. More preferably, the distance h is adjusted within the range of not less than 0.4 mm.
In the creeping discharge spark plug having the above-described first structure, the difference dxe2x88x92D between the outer diameter D of the center electrode and the diameter of the through hole, into which the center electrode is inserted, is preferably set to 0.07 mm or more as measured at a position separated from the tip end of the insulator by 5 mm as measured along the axial direction. The reason will be described below.
The present inventors consider that a corrosion suppression layer is formed through the mechanism as described below. That is, upon generation of spark discharge, gas molecules in the vicinity of the spark discharge gap are ionized; and the thus-produced ions accelerate and hit the discharge surface of the electrode due to a gradient of electrical field created in the gap, so that the metal components of the electrodes are sputtered. The thus-sputtered metal components become oxides immediately and deposit on the surface of the insulator. The deposited oxides form a corrosion suppressing layer.
All of the reaction product formed through oxidation of sputtered metal components does not necessarily contribute to formation of the corrosion suppression layer. A portion of the reaction product accumulates in the clearance between the center electrode and the through hole of the insulator as dust. Further, portions cut from the corrosion suppression layer may enter and accumulate in the clearance as dust. In either case, when the clearance is small, generated dust accumulates in the clearance and fills the clearance densely. In such a case, upon repetition of heating/cooling cycles, the insulator may crack due to difference in thermal expansion between the center electrode made of metal and the insulator made of ceramic.
However, through intensive studies, the present inventors found that when a clearance which is represented by the difference between the outer diameter of the center electrode and the diameter of the through hole of the insulator is set to 0.07 mm or more, dust is prevented from densely filling the clearance. That is, even when dust generated during formation of the corrosion suppression layer enters the clearance between the center electrode and the insulator, the insulator does not crack when subjected to repeated heating/cooling cycles. The reason why the size of the clearance is defined at a position separated from the tip end of the insulator by 5 mm as measured along the axial direction is as follows. That is, the spark plug is typically attached to a cylinder head such that the spark discharge gap; i.e., the tip end of the insulator, faces downward. The dust generated due to formation of the corrosion suppression layer enters the clearance, while being pressed upward by means of combustion pressure. Meanwhile, creeping discharge sparks enter the interior of the insulator. Therefore, the center electrode is consumed in a region to which the sparks reach. As a result, dust present at a position at which the center electrode is hardly consumed and to which influence of heating and cooling reaches easily; i.e., at a position separated from the tip end of the insulator by about 5 mm, is likely to receive the influence of the heating/cooling cycles. Meanwhile, in some cases, the corrosion suppression layer is partially removed by means of creeping discharge sparks, and a phenomenon similar to channeling may occur. Notably, in the above-described spark plug of the present invention, since a reaction product produced through oxidation of sputtered metal components deposits on the removed portion of the corrosion suppression layer to thereby restore it, channeling hardly proceeds to the insulator portion.
Notably, the strength of attack of creeping discharge spark against the insulator; i.e., easiness of occurrence of channeling, changes depending on the polarity of voltage applied to the electrodes for producing spark discharge. Especially, applying voltage for spark discharge such that the center electrode assumes a positive polarity is more advantageous in suppressing channeling than is applying voltage such that the center electrode assumes a negative polarity. When voltage is applied to the electrode such that the center electrode assumes a negative polarity, as described above, the difference dxe2x88x92D between the outer diameter D of the center electrode and the diameter of the through hole, into which the center electrode is inserted, is preferably set to 0.07 mm or more as measured at a position separated from the tip end of the insulator by 5 mm as measured along the axial direction. By contrast, when voltage is applied to the electrode such that the center electrode assumes a positive polarity, only a small amount of dust is generated due to its channeling suppressing effect, and therefore, the difference dxe2x88x92D can be set to 0.03 mm or more (preferably, 0.04 mm or more).
The Ni alloy which forms the electrode base material of the center electrode contains any of Cr, Fe, Cu, Zn, Ti, Ru, V, Co, Nb and Ta as the above-described NTC element. When the above-described NTC oxide semiconductor is formed from these elements, their ionic radiuses become relatively small, so that these elements can easily diffuse and penetrate into the surface of the insulator made of alumina. Thus, the boding strength of the formed corrosion suppression layer is increased, which is effective for stably maintaining the effect of suppressing corrosion against the insulator and the channeling prevention effect.
The above-described effects become remarkable when at least one of Cr, Fe and Cu is employed as an NTC element. In this case, the constituent metal (Ni alloy) of the electrode base material preferably contains Cr; specifically, the Cr content of the Ni alloy is adjusted within a range of 1.5 to 9% by mass. When the Cr content is less than 1.5% by mass, the effect of reducing discharge voltage cannot be attained in some cases. Moreover, when the above is applied to a creeping discharge spark plug, the corrosion suppression function of the layer formed on the surface of the insulator becomes insufficient, so that the channeling prevention effect becomes insufficient. When the Cr content exceeds 9% by mass, the coefficient of thermal conductivity cannot be increased to 17 W/mxc2x7K or higher in some cases. Cr and Fe are more advantageous than other NTC elements, because Cr and Fe can improve the high-temperature strength of the Ni alloy, to thereby achieve simultaneously high-temperature electrode durability and prevention of channeling of the insulator.
The effect of improving the heat transfer property of the electrode can be obtained not only in creeping discharge spark plugs which involve a channeling problem, but also in spark plugs in which creeping discharge along the surface of the insulator does not occur in an ordinary state; e.g., a so-called parallel electrode spark plug in which one side surface of the ground electrode faces the tip end surface of the center electrode.
That is, the present invention provides a spark plug of a second structure comprising:
a center electrode having, at its tip end portion, a consumption-resistant portion made of a noble metal or a composite material containing the noble metal as a predominant component;
an insulator disposed to surround the center electrode; and
a ground electrode disposed such that a side surface of a tip end portion of the ground electrode faces, in parallel, a tip end surface of the center electrode, to thereby form a parallel aerial discharge gap, wherein
an electrode base material, which forms at least a surface layer portion of the center electrode, is formed of an Ni alloy which contains Ni as a predominant component and Cr in an amount of 1.5 to 9% by mass as a secondary component, and has a coefficient of thermal conductivity of 17 to 30 W/mxc2x7K. In this structure, a layer formed on the surface of the insulator does not necessarily participate in suppression of corrosion such as channeling (in the present specification, for the sake of convenience, the layer may be referred to as xe2x80x9ccorrosion suppression layerxe2x80x9d in such a case).
In the above-described structure, when the Cr content of the Ni alloy which forms the electrode base material is less than 1.5% by mass, the oxidation resistance of the electrode base material becomes insufficient, so that a crack stemming from oxidation of the electrode base material is likely to be generated at the junction interface (e.g., welding interface) between the electrode base material and the consumption-resistant portion made of a noble metal and provided at the tip end portion of the center electrode, so that separation of the consumption-resistant portion occurs easily. When the Cr content exceeds 9% by mass, an excessively thick layer containing the NTC semiconductor oxide is formed on the surface of the insulator, so that the resistivity of the surface of the insulator decreases. As a result, sparks are produced at locations other than the regular spark discharge gap; e.g., sparks (so called lateral sparks) are likely to be produced between the side surface of the insulator and the inner circumferential surface of the metallic shell.
In the above-described two structures for spark plugs, the coefficient of thermal conductivity of the constituent metal (Ni alloy) of the electrode base material is set to 17 W/mxc2x7K or higher, because when the coefficient of thermal conductivity is less than 17 W/mxc2x7K, the thermal transfer performance of the electrode deteriorates, and thus durability in terms of electrode consumption cannot be secured. Further, the coefficient of thermal conductivity is limited to not greater than 30 W/mxc2x7K, because when the coefficient of thermal conductivity is to be increased beyond 30 W/mxc2x7K, the Ni content of the Ni alloy must be increased, with the result that the discharge-voltage-decreasing effect or insulator-corrosion-suppressing effect of the layer which originates from the electrode base material and formed on the surface of the insulator becomes insufficient. In view of the above, the Cr content of the Ni alloy is preferably set within the above-described range, more preferably in the range of 2 to 5% by mass.
More preferably, the electrode base material is made of a material which contains Fe in an amount of 1 to 5% by mass. Use of such material further improves the insulator-corrosion-suppressing effect or discharge-voltage-decreasing effect of a formed corrosion suppression layer. The formed corrosion suppression layer contains both Fe and Cr. When the Fe content of the Ni alloy exceeds 5% by mass, the coefficient of thermal conductivity is likely to deviate from the above-described range. When the Fe content of the Ni alloy is less than 1% by mass, the effect obtained through addition of Fe cannot be attained sufficiently. The total content of Fe and Cr is preferably set to 2 to 9% by mass.
Preferably, the Ni alloy which constitutes the electrode base material contains Cr as an essential component and at least one of Fe and Cu as an additional component. In this case, a formed corrosion suppression layer contains Cr as an essential component and at least one of Fe and Cu as an additional component. Cr is an element necessary for securing oxidation resistance of the electrode base material and stabilization of the corrosion suppression layer. Fe and Cu are effective in decreasing discharge voltage. In this case, more preferably, the Ni alloy contains as secondary components Fe in an amount of 1% by mass or more and Cr in an amount of 1.5% by mass or more. When the Fe content is less than 1% by mass, the discharge-voltage-decreasing effect becomes poor, with the result that capacitive discharge voltage increases, and sufficient channeling suppressing effect cannot be expected. When the Cr content is less than 1.5% by mass, the oxidation resistance of the electrode base material and the effect of stabilizing the corrosion suppression layer cannot be secured sufficiently. In this case, the total content of Fe and Cr is preferably set to 2.5 to 9% by mass.
From the viewpoint of suppressing oxidation of the Ni alloy which constitutes the electrode base material, the Cr content is preferably made higher than the Fe content (although the Fe content can be set to 0% by mass, the Ni alloy desirably contains Fe in order to decrease discharge voltage as described above). In this case, more desirably, the ratio of Cr content WCr (% by mass) to Fe content WFe (% by mass), WCr/WFe, is 2 or greater.
Even when the Ni alloy which constitutes the electrode base material of the center electrode contains as a secondary component at least one element selected from among Ru, Zn, V, Co, Nb, Ta and Ti, through formation of a corrosion suppression layer on the surface of the insulator, a channeling suppressing effect can be attained in a similar manner.
The present invention further provides a spark plug of a third structure comprising:
a center electrode:
an insulator disposed to surround the center electrode; and
a ground electrode disposed relative to a tip end portion of the insulator and a tip end portion of the center electrode such that a spark discharge gap is formed between the ground electrode and the tip end portion of the center electrode, and creeping spark discharge along a surface of the tip end portion of the insulator can occur at the spark discharge gap, wherein
an electrode base material which forms at least a surface layer portion of the center electrode is made of an Ni alloy containing Ni as a predominant component and further containing, as a secondary component, an element selected from among Ru, Zn, V, Co, Nb, Ta and Ti.
In the spark plugs having the first through third structures, respectively, the Ni content of the Ni alloy which constitutes the electrode base material is preferably set to 80% by mass or more in order to increase the coefficient of thermal conductivity of the electrode base material to 17 W/mxc2x7K or higher. Further, in order to obtain a remarkable channeling suppressing effect though formation of a corrosion suppression layer (for the first and third structures), or in order to obtain a remarkable effect in improving the thermal transfer property of the electrode (for the second structure), the total content of secondary components of the Ni alloy which constitutes the electrode base material is preferably set to 1.5% by mass or more. Meanwhile, the total content of the secondary components is desirably restricted to not greater than 10% by mass in order to secure sufficiently high consumption resistance of the center electrode.
Next, features which can be commonly added to the spark plugs having the first through third structures, respectively, will be described. First, the center electrode has a structure such a heat-radiation-promoting metal portion made of a material having a coefficient of thermal conductivity higher than that of the electrode base material is embedded within the electrode base material and extends along the axis thereof. By virtue of this configuration, transfer of heat from the tip end portion of the center electrode at which temperature is prone to increase can be promoted effectively, so that the service life of the spark plugs can be increased through suppression of electrode consumption. Here, the side, with respect to the axial direction, on which the tip end surface of the center electrode is located is referred to as the front side, and the side opposite the front side is referred to as the rear side; and the front side of the tip end surface (reference position) of the insulator is considered to be a xe2x80x9c+xe2x80x9d side and the rear side thereof is considered to be a xe2x80x9cxe2x88x92xe2x80x9d side. The tip end of the heat-radiation-promoting metal portion is desirably located within a range of xc2x11.0 mm relative to the tip end surface of the insulator. When the tip end of the heat-radiation-promoting metal portion is retracted into the insulator beyond xe2x88x921.0 mm relative to the reference position, the effect of promoting, by means of the heat-radiation-promoting metal portion, transfer of heat from the tip end potion of the center electrode becomes insufficient, with the result that the electrode may be consumed quickly. When the tip end of the heat-radiation-promoting metal portion is projected from the tip end surface of the insulator beyond +1.0 mm relative to the reference position, upon progress of consumption of the electrode base material, the heat resistance of the tip end portion of the electrode deteriorates, so that the spark plug may quickly reach the end of its service life.
In the above-described structure, the thickness of the electrode base material as measured along a radial direction with respect to the axis and at an axial position separated rearward by 0.5 mm from the tip end surface of the insulator is preferably set to 30% or more the outer diameter of the center electrode at that position. By virtue of this configuration, while efficiently promoting, by the heat-radiation-promoting metal portion, transfer of heat from the tip end portion of the center electrode at which temperature is prone to increase, it is possible to secure sufficiently high durability against electrode consumption due to sparks in the semi-creeping discharge gap at that position.
Moreover, the ground electrode may have a structure such that its surface portion is formed of an electrode base material made of Ni or an Ni alloy, and a heat-radiation-promoting metal portion made of a material having a coefficient of thermal conductivity higher than that of the electrode base material is embedded within the electrode base material and extends along the longitudinal direction of the electrode. This configuration promotes transfer of heat from the ground electrode to thereby enhance durability against consumption. In this case, in the ground electrode, the tip end of the heat-radiation promoting metal portion material is preferably located within the range of 0.5 to 1.0 mm as measured from the tip end surface of the ground electrode. The heat-radiation-promoting metal portion embedded in the center electrode or the ground electrode is preferably made of Cu or a Cu alloy, which is effective for realizing excellent heat radiation property at low cost.
A portion of the ground electrode and/or the center electrode which forms a spark discharge gap may be a consumption-resistant portion which is made of a noble metal or a composite material predominantly containing the noble metal. This configuration effectively suppress an increase in the spark discharge gap due to electrode consumption, so that the service life of the spark plug can be increased. Preferably, the consumption-resistant portion contains, as a predominant component, at least one noble metal selected from Ir, Pt and Ru. Such a consumption-resistant portion can be formed easily by fixing the consumption-resistant portion to the ground electrode and/or the center electrode through any one of laser-beam welding, electron-beam welding and resistance welding.